Compared with liquid crystal displays (LCDs), OLED devices have the advantages of self-luminous property, rapid response speed, wide viewing angle, high brightness, rich color, light weight, low thickness, etc. An OLED device generally includes an anode layer, an emission layer (EML) and a cathode layer and can be divided into a bottom-emission type and a top-emission type according to different configuration of light-emitting surfaces. Top-emission devices have become research focus in recent years as larger aperture ratio can be obtained. A top-emission OLED requires a thin cathode and a reflecting anode to increase the light transmittance, and the thin transparent cathode has large sheet resistance and notable IR drop. Generally, the IR drop of a light-emitting surface of the OLED farther away from a power supply place is more obvious, and hence the OLED device has obvious phenomenon of uneven luminescence.
In order to alleviate the uneven brightness phenomenon of devices, many proposals are presented. In most proposals, auxiliary electrodes that are communicated with the transparent cathode and are connected with each other are additionally provided. The auxiliary electrode may be generally formed of a metal with small electric resistivity, and has a large thickness, sheet resistance of about 1Ω, and reduced IR drop. Therefore, when a power source is applied, the IR drop running through a cathode of the panel is small, and hence the brightness uniformity can be alleviated.
Because the auxiliary electrodes are opaque and light cannot pass through the auxiliary electrodes, the additionally arranged auxiliary electrodes cannot be placed over the EML. Two solutions, namely upper auxiliary electrodes and lower auxiliary electrodes, may be provided according to the fact that the auxiliary electrodes are formed on an array substrate (array backplane) or a color filter (CF) substrate (CF backplane).
As for the upper auxiliary electrode solution, a CF substrate and an OLED substrate are cell-assembled together by means of vacuum pressing, and a conductive layer on spacers makes contact with a cathode under pressure and is deformed. Thus, there are invoked two problems: 1. The deformation of the spacers may result in the breakage of the conductive layer, and the connection between the auxiliary electrodes and the cathode may be broken, so that the pressing strength must be accurately controlled; and 2. Because the contact between the conductive layer on the spacer and the cathode is surface contact, poor contact can be produced.
As for the lower auxiliary electrode solution, in order to avoid the poor contact between the auxiliary electrodes and the cathode, the auxiliary electrodes are formed in non-luminous regions on the cathode. Thus, there is invoked one problem: the positioning accuracy requirement of the auxiliary electrodes can be easily satisfied by the traditional exposure process, but OLED materials are very sensitive to moisture and water vapor and the thin-film transistor (TFT) etching process is incompatible. In another aspect, the thin cathode metal can be also easily over-etched. When the auxiliary electrodes are formed by fine metal mask (FMM) evaporation, as for small-size panels, the auxiliary electrodes can be easily obtained by evaporation, but the problem of long evaporation time can be caused because the auxiliary electrodes are generally thick. What is worse, when the size of the panel is increased, the correspondingly required FMM is larger, and hence the alignment problem can be caused by the gravity effect to the mask.